Sport (Harriet the Spy Adventures)

Eleven-year-old Sport Rocque is living a happy life, keeping his father's absentmindedness under control, and managing the family budget. When Kate, Sport's new-and nice-stepmother enters the picture, things couldn't be better. Then comes the news: Sport's wealthy grandfather has just died and Sport is a multimillionaire.
But millions of dollars equals millions of problems, as Sport soon discovers when his mother returns and kidnaps him to double her share of the inheritance! Life at the Plaza Hotel is no fun when you're a prisoner. Will Sport manage to return his life to normal?